Mix shader is not working
Well, someone should write an answer, because long ago, I got stuck on such things. (Of course I never get stuck now :P)

If the red disk is supposed start off solid red and fade into invisibility at 50 frames in, then it works fine.  It may not look so in the 3D view, unless you have VIewport Shading set to  Rendered mode (the small button just right of the Object Mode / Edit Mode button.)  Textured, Material and Solid view shading modes won't show animation of textures.